1. Field of the Invention
This invention relates to a power supply device for an electromotive railcar which insulates a three-phase alternating current (AC) voltage from a direct current (DC) voltage from an external electric line and, more particularly to a power supply device having DC/AC/DC converters.
2. Discussion of the Background
FIG. 5 is a circuit diagram of a conventional power supply device for an electromotive railcar.
In FIG. 5, the power supply device obtains a DC voltage from an electric power line 1. The DC voltage charges an electrolytic capacitor 8 via a pantograph 2, a fuse 3, a contactor 4 (contact breaker), a DC reactor 5 and an initial charge resistor 7. When the electrolytic capacitor 8 is charged to a predetermined voltage, a conducting thyristor 6 connected in parallel with the initial charge resistor 7 turns on. Then a three-phase inverter 9 is operated.
The three-phase inverter 9 generates a three-phase AC voltage 13 on the basis of the DC voltage from the electric power line 1. Output waveforms of the three-phase inverter 9 are well-known PWM (Pulse Width Modulation) sinewaves including many higher harmonics.
Therefore, the higher harmonics are removed by passing the voltage signal through an AC filter comprising an AC reactor 10 for smoothing and an AC capacitor 11, and then a commercial power signal with 50 Hz or 60 Hz and 200V is obtained. The electrolytic capacitor 8 and the three-phase inverter 9 are coupled to a ground 14.
The commercial power signal is mainly used for operating air conditioners and lighting on railcars. Moreover, the commercial power signal is insulated through an insulating transformer 12 with a commercial carrier frequency for the purpose of insulating the three-phase AC voltage 13 from the DC voltage from the electric power line 1.
After a commercial voltage (for example 270V ) is obtained, the commercial voltage is insulated by the insulating transformer 12 whose carrier frequency is a commercial frequency of 50 Hz or 60 Hz. A control device disclosed in Japanese Patent Disclosure (kokai) No. 7-31156 is applicable for the controller of the three-phase inverter 9.
However, there are some problems in the conventional power supply device of FIG. 5.
First, the insulating transformer 12 becomes heavy and large, because the carrier frequency is a relatively low commercial frequency. Moreover, the insulating transformer 12 causes noise of the commercial frequency.
Further, the same voltage as that of the electric power line 1 is applied to the three-phase inverter 9, the AC reactors 10 and the AC capacitors 11. Therefore, the conventional power supply device must be suitably insensitive to voltage fluctuations and becomes collectively large and costly.
Furthermore, load fluctuation from load objects, such as air conditioners or lighting, causes an adverse influence on the current of the DC voltage from the electric power line 1. Therefore, the electrolytic capacitor 8 charged with the DC voltage must be large enough to remove a ripple wave (50 Hz or 60 Hz) caused by the load fluctuation.
Accordingly, one object of this invention is to provide a miniaturized, light weight, low noise and low price power supply device for an electromotive railcar. The present invention provides a power supply device for an electromotive railcar, and comprises a first capacitor connected to receive the DC voltage for outputting a first DC voltage. The power supply device also comprises a DC/AC/DC converter that includes an inverter bridge having power transistors connected to the first capacitor in parallel, an insulating transformer with high carrier frequency having an primary winding connected to an output of the inverter bridge, and a rectifier circuit connected to a secondary winding of the insulating transformer to receive a second DC voltage. The power supply device further comprises a three-phase inverter having a bridge circuit of power transistors for generating a three-phase AC voltage on the basis of the second DC voltage.